Chiral structures have been found as the lowest-energy isomers of bare (Au 28 and Au 55 ) and thiol-passivated ͓Au 28 (SCH 3 ) 16 and Au 38 (SCH 3 ) 24 ] gold nanoclusters. The degree of chirality existing in the chiral clusters was calculated using the Hausdorff chirality measure. We found that the index of chirality is higher in the passivated clusters and decreases with the cluster size. These results are consistent with the observed chiroptical activity recently reported for glutahione-passivated gold nanoclusters, and provide theoretical support for the existence of chirality in these compounds.Detailed knowledge of the lattice structure, shape, morphology, surface structure, and bonding of bare and passivated gold clusters is fundamental to predict and understand their electronic, optical, and other physical and chemical properties. This information is essential to optimizing their utilization as novel nanocatalysts, 1 and as building-blocks of new molecular nanostructured materials, 2 with potential applications in nanoelectronics 3 and biological diagnostics. 4 An effective theoretical approach to determine gold cluster structures is to combine genetic algorithms and many-body potentials ͑to perform global structural optimizations͒, and first-principles density functional theory ͑to confirm the energy ordering of the local minima͒. Using this procedure we recently found many topologically interesting disordered gold nanoclusters with energy near or below the lowestenergy ordered isomer. 5-8 The structures of these clusters showed low spatial symmetry or no symmetry at all, opening the possibility of having distinct electronic and optical properties in such systems. In other studies on passivated gold nanoclusters, 9,10 we also found that the effect of a methylthiol monolayer ͑24 SCH 3 molecules͒ on a truncatedoctahedron ͑with fcc geometry͒ Au 38 cluster is strong enough to produce a dramatic distortion on the gold cluster, resulting in a disordered geometry for the most stable Au 38 (SCH 3 ) 24 passivated cluster.Although the calculated structure factors of the disordered gold clusters were in qualitative agreement with the data obtained from x-ray powder diffraction on experimental samples, 6,7 the direct confirmation of the existence of bare and thiol-passivated gold nanoclusters with low or no spatial symmetry had not been possible due to the lack of enough experimental resolution for clusters in the size range of 1-2 nm. 11-13 Nevertheless, in a recent study using circular dichroism, Shaaff and Whetten ͑SW͒ 14 found a strong optical activity in the metal-based electronic transitions ͑across the near-infrared, visible and near ultraviolet regions͒ of sizeseparated glutathione-passivated gold clusters in the size range of 20-40 Au atoms. SW pointed out that the most plausible interpretation of these results is that the structure of the metal-cluster core of the gold-glutathione cluster compounds would be inherently chiral. Moreover, since the most abundant cluster in the experimental samples corresponds...

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Heat capacities of NaN , N = 13,20, 55, 135, 142, and 147, clusters have been investigated using a many-body Gupta potential and microcanonical molecular dynamics simulations. Negative heat capacities around the cluster melting-like transition have been obtained for N = 135, 142, and 147, but the smaller clusters (N = 13, 20, and 55) do not show this peculiarity. By performing a survey of the cluster potential energy landscape (PEL), it is found that the width of the distribution function of the kinetic energy and the spread of the distribution of potential energy minima (isomers), are useful features to determine the different behavior of the heat capacity as a function of the cluster size. The effect of the range of the interatomic forces is studied by comparing the heat capacities of the Na55 and Cd55 clusters. It is shown that by decreasing the range of the many-body interaction, the distribution of isomers characterizing the PEL is modified appropriately to generate a negative heat capacity in the Cd55 cluster.

We predict general trends for surface segregation in binary metal clusters based on the difference between the atomic properties of the constituent elements. The energetically most favorable site for a guest atom on a pure metal cluster is determined considering the attractive and repulsive contributions of the cohesive energy of an atom in the cluster. It is predicted that for adjacent elements in a period of the periodic table, the bimetallic system would be more stable if the component with smallest valence electron density is placed on the surface. On the other hand, in bimetallic clusters built with elements of only one group, the trend to be in the volume ͑of the atomic component with smaller core density͒ will be higher for that cluster with atomic components most separated in the group. Such chemical ordering trends in the lowest energy configurations of Pt-Au, Pt-Pd, and Pt-Ni binary alloy clusters are verified for their 561 atom systems through a simulated annealing process. Some of our atomistic predictions are verified through quantum mechanical calculations.

To dispose of atomic oxygen, it is necessary the O activation; however, an energy barrier must be overcome to break the O-O bond. This work presents theoretical calculations of the O adsorption and dissociation on small pure Au and Ag and bimetallic AuAg (n + m ≤ 6) clusters using the density functional theory (DFT) and the zeroth-order regular approximation (ZORA) to explicitly include scalar relativistic effects. The most stable AuAg clusters contain a higher concentration of Au with Ag atoms located in the center of the cluster. The O adsorption energy on pure and bimetallic clusters and the ensuing geometries depend on the spin multiplicity of the system. For a doublet multiplicity, O is adsorbed in a bridge configuration, whereas for a triplet only one O-metal bond is formed. The charge transfer from metal toward O occupies the σ* antibonding natural bond orbital, which weakens the oxygen bond. The Au (A) cluster presents the lowest activation energy to dissociate O, whereas the opposite applies to the AuAg (A) system. In the O activation, bimetallic clusters are not as active as pure Au clusters due to the charge donated by Ag atoms being shared between O and Au atoms.

ABSTRACT:The Au(I)-Au(I) closed-shell or aurophilic attraction has been the subject of interest in the experimental and theoretical chemistry fields, due to the intriguing properties associated to it. The presence of phosphorescence in ''aurophilic'' compounds has been addressed to a wide range of applications, but it has not yet been fully understood. A theoretical study on the electronic and phosphorescent properties of the following series of dinuclear gold complexes has been performed: [Au 2 (dmpm) (i-mnt)] (1), [Au 2 (l-Me-TU) (l-dppm)] (2), and [Au 2 (l-G)(l-dmpe)] (3). Full geometry optimizations at the second-order Møller-Plesset perturbation theory (MP2) were carried out for each of the species. These calculations made evident that, at the groundstate geometry, the Au(I) cations allocated at the center of the ring show a short Au-Au distance below the sum of the van der Waals radii, at the range of the aurophilic attraction. An intermolecular Au(I)-Au(I) closed-shell attraction for a pair of the systems under study is found. This attraction is comparable to that of the hydrogen bonds. The phosphorescent properties experimentally observed for this series were also characterized through ab initio techniques. The obtained results allow to fit reasonably

Irreversible losses and heat transport in a magnetohydrodynamic flow of a viscous, steady, incompressible, and fully developed couple stress Al 2 O 3-water nanofluid through a sloping permeable wall channel with porous medium and under the effect of radiation heat flux and slip were analyzed. The fundamental equations were solved numerically by using Runge-Kutta together with the shooting technique and the results were in qualitative agreement with an exact solution obtained for a limit case. The impacts of couple stress, Darcy number, solid nanoparticle concentrations, conduction-radiation parameter, Hartmann number and hydrodynamic slip on flow, temperature, heat transport, and entropy production were examined. It was possible to achieve values of minimum entropy production not yet reported in previous studies. In this way, optimal values of couple stress and slip were obtained. The heat transport was also explored and optimal values of slip flow and conduction-radiation parameter with maximum heat transfer were found. Finally, in addition to the alumina, the distributions of velocity, temperature, and entropy generation in TiO 2-water and Cu-water were presented for different solid nanoparticle concentrations. It was obtained that the local entropy of TiO 2-water was lower than Cu-water and Al 2 O 3-water in the channel bottom region while it was greater in the upper region.

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